Many investigations of fatigue of metals under actual stresses have been carried out recently. Most of them have been done in the case where only the amplitudes of repeated stresses are varying with time. In the case of actual stress spectra of machine members, however, not only the amplitudes of repeated stresses but also the mean stresses fluctuate, and extremely complicated stress patterns are repeated.As one of the investigations of those phenomena, the fatigue tests of the low-carbon steels have been carried out by the authors in the case where the repeated bending stress amplitudes are constant and the mean bending stresses fluctuate cosinusoidaly with time.In order to analyze the varying stress spectra, and to discuss the strength and lives of metal members under those fluctuating stresses, it is necessary to measure the values of actual stresses induced on the test piece, For this purpose, the bending moments applied on the test pieces have been measured from the elastic strain of the driving shaft or of the jaw gripping of the test piece with the electrical resistance strain meter, the electromagnetic oscillograph, and the vacuum tube volt meter.Besides in order to examine the patterns of the repeated stresses, they have been monitored with the synchroscope, then it is ascertained that those patterns are exactly sinusoidal, and it is also ascertained with electromagnetic oscillograph that the mean stresses fluctuate surely cosinusoidaly with time.As the results of the tests, it has been made clear that, in the case of the repeated bending stresses with the varying mean bending stresses, the endurance limits and the fatigue lives of the repeated bending stresses are considerably reduced in comparison with those in the case of the repeated bending stresses with the constant mean bending stresses.